Additive manufactured strain gauge on component surfaces for predictive failure monitoring

ABSTRACT

An additively manufactured strain gauge resides on the surface of a component to monitor component fatigue. The strain gauge is additively manufactured, and applied to the curvature of the component surface through either a flexible substrate or through direct printing.

BACKGROUND

This application relates generally to aircraft components andspecifically to strain gauge monitoring of components.

Heated metallic or composite components in aircraft, such as floorpanels, rotor blades, wing leading edges, stabilizers, and heated inletscan be subject to a high amount of stress and fatigue while beingoperated. These components should be monitored to ensure they do notfail while in flight. Ideally, upcoming failure should be predictableand the devices used to monitor such stresses should outlast thecomponents themselves.

Conventional strain gauges used for component predictive failuremonitoring have short lifespans. Conventional strain gauge, for example,use fine wires or very thin etched circuits that can have shortoperating lives, particularly in high strain environments, that do notlend themselves for use with predictive failure monitoring systems andalgorithms.

SUMMARY

This application discloses strain gauges that are additivelymanufactured to provide sufficient operating lives that make them usefulwith predictive failure monitoring systems and algorithms.

In a first embodiment, an assembly includes a component have a topsurface, a strain gauge additively manufactured thereon, and acontroller. The strain gauge includes an additively manufactured portioncomprising a conductive ink defining a sensing portion and one or moreterminals and an encapsulating material, and one or more flex leadsconnected to the one or more terminals. The controller is electricallyconnected to the printed portion through the one or more flex leads.

In a second embodiment, a system for monitoring fatigue of a componentincludes an additively manufactured strain gauge on a first surface of acomponent, a controller for reading and analyzing data produced by theadditively manufactured strain gauge, an electrical connection betweenthe additively manufactured strain gauge and the controller, and apredictive program on the controller for predicting when the componentwill fail based on data from the additively manufactured strain gauge.

In a third embodiment, a method of making a strain gauge on a componentincludes additively manufacturing a gauge onto a first surface of thecomponent and electrically connecting the gauge to an externalcontroller.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top-down view of an additively manufactured strain gauge.

FIG. 2 is a perspective, cut-away view of a heated floor panel assemblywith an additively manufactured strain gauge.

FIG. 3 is a perspective, cut-away view of a rotor blade component withan additively manufactured strain gauge.

DETAILED DESCRIPTION

An additively manufactured strain gauge will have a longer lifespan thana conventional wire or etched strain gauge due to the flexible nature ofinks that are additively manufactured. This type of strain gauge couldconceivably be printed directly into an aircraft component duringmanufacturing, allowing for a longer lifespan of the strain gauge deviceand extensive monitoring of stresses on the aircraft component. Examplesof such aircraft components include but are not limited to floor panels,rotor blades, wing leading edges, horizontal stabilizers, verticalstabilizers, heated inlets, or other composite or metallic componentsfor use in aircraft that are heated in use. A person of ordinary skillin the art would recognize that other aircraft components would also becompatible with the disclosed additively manufactured strain gauge.

A strain gauge with a long lifespan allows for long term componentmonitoring, prediction of component failure, and the creation of apredictive failure model based on raw strain gauge data. These modelscould allow for predictive failure of components where conventionalstrain gauges would fail too early to create such predictive models.

FIG. 1 is a top-down view of additively manufactured strain gauge 10.Strain gauge 10 has first side 12, second side 14, sensing portion 16,terminals 18, 20, and leads 22, 24 connecting strain gauge 10 tocontroller 26. Strain gauge 10 resides on dielectric layer 28.

Strain gauge 10 is a three-dimensional additively manufactured devicemade of a conductive ink within a resistivity range of between 0.01 and20 ohms per square per MIL (where MIL=0.001 inch), or ideally between 1and 15 ohms per MIL. The conductive ink can be, for example, carbonloaded, nano-carbon, or silver loaded ink. Some examples of conductiveinks specialized for printed electronics are DuPont® PE410, DuPont®PE671, DuPont® PE873, Methode 9101, Methode 3800-series inks, EngineeredConductive Materials CI-2001, and Engineered Conductive MaterialsCI-2042. The conductive ink of strain gauge 10 is formulated to allowhighly detailed precision printing, and maintain a high resistancewithout bleeding between adjacent additively manufactured lines. Straingauge 10 is additively manufactured onto assembly 10 through a printingprocess such as screen printing, ink-jet, or aerosol-jet printing.

Sides 12, 14, and sensing portion 16 of strain gauge 10 are all made ofconductive ink. Side 12, 14, define the outer limits of strain gauge 10,while sensing portion 16 is the sensing portion of strain gauge 10.

Terminals 18, 20, reside on opposite ends of sensing portion 16, andserve as “ends” to strain gauge 10, across which voltage is inputted andoutputted, and change in resistance is measured. Leads 22, 24 createelectrical connections between terminals 18, 20 and controller 26,allowing for effectively monitoring of strain of the component on whichstrain gauge 10 resides. Leads 22, 24 can be manufactured byconventional methods or alternatively additively manufactured along withthe rest of strain gauge 10.

Dielectric layer 28 serves as a carrier for first side 12, second side14, sensing portion 16, and terminals 18, 20, and prevents theseconductive parts from shorting each other.

Strain gauge 10 is a sensor whose resistance varies with applied force.Strain gauge 10 converts force (pressure, tension, weight, etc.) into achange in electrical resistance which can be measured between terminals18 and 20. When external forces are applied to a component on whichstrain gauge 10 is applied, stress/strain results. Stress is thecomponent's internal resisting force, while strain is the displacementand deformation that occur within the component.

In practice, an excitation voltage is applied to strain gauge 10 throughflex leads attached to terminals 18, 20, and a voltage reading is taken.Typical input voltages can be between 5 V and 12 V, whereas typicaloutput readings can be in millivolts. Different applications placedifferent requirements on strain gauge 10. In most cases, theorientation of strain gauge 10 is significant, and can affect readingsfrom strain gauge 10. Typically, strain gauge 10 is laid across an axison a surface of the component so that strain gauge 24 detects fatigueand stress typical to that component.

Strain gauge 10 can be manufactured in two ways: onto a flexiblesubstrate, which is then applied to a component surface, or directlyprinted onto the component surface.

In the first method, a flexible substrate is prepared. The flexiblesubstrate must be able to conform to the curvature of the componentsurface to which strain gauge 10 will be applied. The substrate can be,for example, neoprene, TPU, urethane, fabric, or other materials. Insome instances, the substrate must be cleaned or cured before printingusing conventional curing methods.

The substrate must be compatible with both the component and theconductive ink. For instance, the flexible substrate must be able towithstand heating occurring with the component, and maintain adhesion tothe component. Additionally, the flexible substrate should be erosionresistant so that strain gauge 10 on the flexible substrate stays on thecomponent for the component lifetime. This is highly dependent on thespecific component and ink chosen. For example, if the component is theleading edge of a wing, the flexible substrate must be able to withstandlight, temperature, and weather external to the aircraft. In contrast,if the component is a heated floor panel, the flexible substrate needsto withstand the heater within the floor panel and the pressure ofpassengers stepping on the panel.

Next, the conductive ink is printed onto the substrate. Typically,ink-jet, aerosol-jet, or screen printing can be used depending on thetype of ink chosen. For ink-jet and aerosol-jet methods, the print headshould be moveable at least on (x, y, z) axes and programmable with thegeometric pattern specific to the component on which the strain gaugewill be applied. The specific print heat and printing method will bedependent on the exact ink formulations and requirements set forth bythe manufacturer of the ink. A popular aerosol-jet equipmentmanufacturer, Optomec, has developed equipment and parametersspecialized for precise application of very fine features at wellcontrolled thicknesses that facilitate the subject invention. Printinghead temperatures, flow rates, pressures, and orifice sizes are selectedbased on the ink being printed as well as the substrate to which the inkis applied. In a similar manner, ink-jet equipment manufacturers haveoptimized parameters of printing heads to allow precision printing ofconductive inks. Ink-jet equipment has historically been utilized in a2D manner, but has recently been adapted to 3D printing equipment usedfor additive manufacturing, such as equipment manufactured by Ultimakeror by means of attaching an ink-jet printing head to a numericallycontrolled robotic arm.

The printing is accomplished in an additive manner, meaning the printhead takes multiple passes before a desired thickness of ink is reachedin the correct geometric pattern, which matches the curvature of thecomponent.

Printed strain gauge 10 should have a thickness of approximately between20 and 300 microns, or preferably between 50 and 100 microns. Multiplepasses are done by the print head when applying the conductive ink.Multiple passes allows for slow build of the conductive ink to thecorrect thickness and geometric pattern. Additionally, multiple passesallows for tailoring of the conductive ink on certain portions of thecomponent surface. For instance, ink with a higher thickness anddiffering resistance may be printed on a first portion of the componentcompared to a second portion of the component.

After printed, the ink is cured, and leads 12, 14, are connected toterminals 18, 20. The curing process of additively manufactured straingauge 10 depends on the type of ink used. In some instances, the inkwill air dry. In other instances, heat, infrared exposure, UV exposure,or other methods must be used to set and cure the conductive ink.

After strain gauge 10 is printed onto the substrate, it is encapsulatedwith a dielectric material, such as acrylic, neoprene, polyurethane,polyimide, silicone, or an epoxy-fiberglass matrix, to prevent erosionand electrical shorting. A typical encapsulating material with highdielectric strength, such as polyimide (DuPont Kapton®), may only berequired to be 0.001″ thick while materials with lower dielectricstrength, such as polyurethane or neoprene rubber, may be as thick as0.015-0.020″. The encapsulating material can then be cured throughconventional methods.

Finally, strain gauge 10 is applied to the component surface with anadhesive such as a pressure sensitive adhesive, epoxy, or cementadhesive, depending on the component and environment requirements. Theflexible substrate allows for conforming of strain gauge 50 to curvatureof the component surface without creating unnecessary stresses withinstrain gauge 50. The use of a flexible substrate allows strain gauge 10to form to the shape of the surface of the component to which it isapplied.

In the second method, the conductive ink is additively manufactureddirectly onto the component surface. In some instances, where thecomponent is a conductive material (e.g., metallic), an intermediarydielectric layer must first be applied to the surface of the componentbefore the conductive ink is printed. This layer may be a supplementalsubstrate bonded to the surface, but preferably it is a non-conductiveink that is applied via the same method being used to print the straingauge.

If the conductive ink is printed directly onto the component surface,the printing method used must allow for a print head that can move inthree dimensions and navigate the geometry of the component surfacewhile printing. Like the first method, the print head will make multiplepasses until the resistance and thickness of strain gauge 50 is correct.Methods such as screen printing, ink-jet or aerosol-jet printing can beused, the preferred method would be selected based on the complexity ofthe shape on which strain gauge 50 is being printed. The printingprocess is similar to that described in reference to the firstembodiment. Once printed, strain gauge 50 must be electricallyconnected, encapsulated, and cured as discussed above.

In some instances, where the assembly surface is electrically conductive(metallic), this necessitates the use of an intermediary dielectriclayer between the assembly and additively manufactured strain gauge 10,such as a non-conductive ink like DuPont BQ10 or ME777, or an integrallybonded layer such as polyimide (Kapton®) or an epoxy-fiberglass. Thetypical thickness of a dielectric layer depends on the dielectricstrength of the material and as a result may vary between 0.0005″ and0.010″ thick. The dielectric layer is not necessary for certain types ofcomposite surfaces. This dielectric layer is thin, and acts as aninsulator and adhesive between the component surface and the additivelymanufactured strain gauge. The dialectic layer, like the flexiblesubstrate in the first embodiment, must be able to withstandtemperatures, light, and other environmental factors so that additivelymanufactured strain gauge 10 maintains its adhesion to the component.

In either embodiment, strain gauge 10 adheres to and matches thegeometry of the surface of the component to which it is applied. Thisallows for greater fatigue resistance over the lifespan of the componentand strain gauge 10. Moreover, multiple applications of strain gauge 10conductive ink allows for varying thickness and resistance of straingauge 10 as needed on the component. Various embodiments of strain gauge10 used in components are discussed with reference to FIGS. 2 and 3.

One embodiment includes a heated floor panel as depicted in FIG. 2. FIG.2 is a perspective, cut-away view of heated floor panel assembly 30 withadditively manufactured strain gauge 10. Heated floor panel assembly 30has bottom surface 32 and top surface 34. Assembly 30 includes firststack of structural layers 36, core layer 38, heater layer 40, secondstack of structural layers 42, strain gauge 10 additively manufacturedinside assembly 30, flex leads 46 connected to strain gauge 10, skinlayer 48, and controller 26.

Bottom surface 32 of heated floor panel assembly 30 is attached to anaircraft, while top surface 34 faces the inside of an aircraft cabin orother floor area needing to be heated. Components 36, 38, 40, 42, and 10are arranged between bottom surface 32 and top surface 34. Heated floorpanel assembly can optionally include additional structural layersand/or a skin layer, such as layer 48 which covers top surface 34 ofassembly 30. Strain gauge 10 is nestled between second stack ofstructural layers 42 and skin layer 48.

In heated floor panel assembly 30, first and second stacks of structurallayers 36 and 42 provide reinforcement to assembly 30. Core layer 38provides impact resistance to assembly 30, and carries shear loads tostiffen floor panel assembly 30. Heating layer 40 can include anelectrical heating element embedded on or in heating layer 40. Heatinglayer 40 can be used to control the temperature of top surface 34 ofassembly 30, which can be installed, for example, in an aircraft cabinor cockpit. Additional and/or alternative structural layers can also beadded in other embodiments.

Strain gauge 10 is additively manufactured inside heated floor panelassembly 30, between second stack of structural layers 42 and skin layer48. Strain gauge is manufactured as described in reference to FIG. 1.Like strain gauge 10 in FIG. 1, strain gauge 10 is made of a conductiveink additively manufactured on the surface of assembly 30 and conformingto the geometry of assembly 30. Similarly, strain gauge 10 can reside ona flexible substrate which is attached to the curvature of assembly 30,or can be directly additively manufactured on assembly 30.

Skin layer 48 covers assembly 30 and protects strain gauge 10. Skinlayer 48 can be, for example, a layer designed to absorb impact toassembly 30, such as a metallic or composite layer adhered to the outersurface of assembly 30.

Controller 26 is in communication with strain gauge 10 via flex leads46. Controller 26 allows for reading of data from strain gauge 10 and isin communication with a controller capable of analyzing data to create apredictive model to determine when assembly 30 will fail or needreplacement based on strain detected by strain gauge 10.

Controller 26 can include one or more processors and controller-readablememory encoded with instructions that, when executed by the one or moreprocessors, cause controller 26 to operate in accordance with techniquesdescribed herein. Examples of the one or more processors include any oneor more of a microprocessor, a digital signal processor (DSP), anapplication specific integrated circuit (ASIC), a field-programmablegate array (FPGA), or other equivalent discrete or integrated logiccircuitry. Controller-readable memory of controller 26 can be configuredto store information within controller 26 during operation. Thecontroller-readable memory can be described, in some examples, ascontroller-readable storage media. In some examples, acontroller-readable storage medium can include a non-transitory medium.The term “non-transitory” can indicate that the storage medium is notembodied in a substrate wave or a propagated signal. In certainexamples, a non-transitory storage medium can store data that can, overtime, change (e.g., in RAM or cache). Controller-readable memory ofcontroller 26 can include volatile and non-volatile memories. Examplesof volatile memories can include random access memories (RAM), dynamicrandom access memories (DRAM), static random access memories (SRAM), andother forms of volatile memories. Examples of non-volatile memories caninclude magnetic hard discs, optical discs, floppy discs, flashmemories, or forms of electrically programmable memories (EPROM) orelectrically erasable and programmable (EEPROM) memories. Controller 26can be a stand-alone device dedicated to the operation of the catalyticoxidation unit, or it can be integrated with another controller.

FIG. 3 is a perspective, cut-away view of rotor blade component 50 withadditively manufactured strain gauge 10. Component 50 has inner surface52 and outer surface 54. Component 50 includes inner plies 56, heaterlayer 58, outer erosion shield 60, strain gauge 10, leads 64, andnonconductive outer ply 66.

Component 50 is a rotor blade for aircraft with a curved surface. Innerplies 56 make up the internal structure of component 50, providingstructural support. Heater layer 58 internally heats component 50, andcan be, for example, a metallic or carbon-based heating element.Nonconductive outer ply 56 protects and insulates heater layer 58 fromstrain gauge 10 or other external items. Erosion shield 60 protects allof component 50, including string gauge 10, from the externalenvironment.

Strain gauge 10 is an additively manufactured strain gauge on the curvedsurface of component 50. Like strain gauge 10 in FIG. 2, strain gauge 10is made of a conductive ink additively manufactured on the surface ofcomponent 50 and conforming to the geometry of component 50. Similarly,strain gauge 10 can reside on a flexible substrate which is attached tothe curvature of component 50, or can be directly additivelymanufactured on component 50. Leads 64 connect strain gauge 10 to anelectrical source and allow for communication with a controller asstrain gauge 10 monitors strain on component 50.

The creation of an additively manufactured strain gauge on a componentallows for predictive failure monitoring of that component. Indetermining a predictive model, stresses applied to a component aremonitored with an additively manufactured stress gauge. In this step, avoltage is applied to the strain gauge. The resulting output voltage ismeasured, and a change in resistance is detected and use to calculatethe strain applied to the component. This data regarding the stresses istransferred to a controller.

This allows the controller to predict failure of the component based onstresses detected over time. Ultimately, this allows for compiling of amodel determining when to replace the component based on the analysis.Models that can predict component fatigue allow for regular maintenanceand replacement schedules to be enforced.

Additively manufactured strain gauges have longer life than traditionalmetallic components due to the flexible nature of conductive ink and itscomponents, in addition to flexible substrates. Printed strain gaugeshave longer lifespans than traditional wire or etched strain gauges,which allows for long term monitoring of component predictive failuremonitoring.

Discussion of Possible Embodiments

The following are non-exclusive descriptions of possible embodiments ofthe present invention.

An assembly includes a component having a top surface, a strain gaugeadditively manufactured thereon, and a controller. The strain gaugeincludes an additively manufactured portion comprising a conductive inkdefining a sensing portion and one or more terminals and anencapsulating material, and one or more flex leads connected to the oneor more terminals. The controller is electrically connected to theprinted portion through the one or more flex leads.

The assembly of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations and/or additional components:

The component is selected from the group consisting of a rotor blade, afloor panel, a wing leading edge, a horizontal stabilizer, a verticalstabilizer, and a heated inlet.

The additively manufactured portion comprises a conductive ink selectedfrom the group consisting of carbon loaded, nano-carbon load, andnano-silver loaded inks.

The additively manufactured portion comprises a plurality of lines, eachof the plurality of lines having a thickness between 20 and 300 microns.

Each of the plurality of lines has a thickness between 50 and 100microns.

The additively manufactured portion has a resistivity of between 0.01and 20 ohms per square per MIL.

The additively manufactured portion has a resistivity of between 1 and15 ohms per square per MIL.

The additively manufactured strain gauge is printed directly on thesurface of the component.

The additively manufactured strain gauge is printed on a flexiblesubstrate that is attached to the surface of the component.

The additively manufactured strain gauge is printed on a dielectriclayer residing on the surface of the component.

A system for monitoring fatigue of a component includes an additivelymanufactured strain gauge on a first surface of a component, acontroller for reading and analyzing data produced by the additivelymanufactured strain gauge, an electrical connection between theadditively manufactured strain gauge and the controller, and apredictive program on the controller for predicting when the componentwill fail based on data from the additively manufactured strain gauge.

A method of making a strain gauge on a component includes additivelymanufacturing a gauge onto a first surface of the component andelectrically connecting the gauge to an external controller.

The method of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations and/or additional components:

Additively manufacturing the gauge comprises printing multiple layers ofthe gauge.

Additively manufacturing the gauge comprises printing with a carbonloaded or silver loaded ink.

Additively manufacturing the gauge comprises printing the gauge duringmanufacture of the component.

Electrically connecting the gauge comprises attaching one or more leadsbetween the gauge and the external controller.

Encapsulating the strain gauge.

Additively manufacturing the gauge comprises screen printing, ink-jetprinting, or aerosol printing the gauge.

Additively manufacturing the gauge comprises printing with a carbonloaded or silver loaded ink.

While the invention has been described with reference to an exemplaryembodiment(s), it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment(s) disclosed, but that theinvention will include all embodiments falling within the scope of theappended claims.

1. An assembly comprising: a component having a surface; a strain gaugeadditively manufactured thereon, the strain gauge comprising: anadditively manufactured portion comprising: a conductive ink defining asensing portion and one or more terminals; and an encapsulatingmaterial; and one or more flex leads connected to the one or moreterminals; and a controller electrically connected to the additivelymanufactured portion through the one or more flex leads.
 2. The assemblyof claim 1, wherein the component is selected from the group consistingof a rotor blade, a floor panel, a wing leading edge, a horizontalstabilizer, a vertical stabilizer, and a heated inlet.
 3. The assemblyof claim 1, wherein the conductive ink selected from the groupconsisting of carbon loaded, nano-carbon load, and nano-silver loadedinks.
 4. The assembly of claim 1, wherein the additively manufacturedportion comprises a plurality of lines, each of the plurality of lineshaving a thickness between 20 and 300 microns.
 5. The assembly of claim4, wherein each of the plurality of lines has a thickness between 50 and100 microns.
 6. The assembly of claim 1, wherein the additivelymanufactured portion has a resistance of between 0.01 and 20 ohms persquare per MIL.
 7. The assembly of claim 6, wherein the additivelymanufactured portion has a resistance of between 1 and 15 ohms persquare per MIL.
 8. The assembly of claim 1, wherein the additivelymanufactured strain gauge is printed directly on the surface of thecomponent.
 9. The assembly of claim 1, wherein the additivelymanufactured strain gauge is printed on a flexible substrate that isattached to the surface of the component.
 10. The assembly of claim 1,wherein the additively manufactured strain gauge is printed on adielectric layer residing on the surface of the component.
 11. A systemfor monitoring fatigue of a component comprising: an additivelymanufactured strain gauge on a first surface of a component; acontroller for reading and analyzing data produced by the additivelymanufactured strain gauge; an electrical connection between theadditively manufactured strain gauge and the controller; and apredictive program on the controller for predicting when the componentwill fail based on data from the additively manufactured strain gauge.12. A method of making a strain gauge on a component comprising:additively manufacturing a gauge onto a first surface; applying thegauge onto the component; and electrically connecting the gauge to anexternal controller.
 13. The method of claim 12, wherein the firstsurface comprises a surface on the component.
 14. The method of claim12, wherein the first surface comprises a flexible substrate, andwherein applying the gauge onto the component comprises adhering theflexible substrate to the component.
 15. The method of claim 12, whereinadditively manufacturing the gauge comprises printing the gauge duringmanufacture of the component.
 16. The method of claim 12, whereinelectrically connecting the gauge comprises attaching one or more leadsbetween the gauge and the external controller.
 17. The method of claim12, further comprising encapsulating the gauge.
 18. The method of claim17, wherein additively manufacturing the gauge comprises screenprinting, ink-jet printing, or aerosol printing the gauge.
 19. Themethod of claim 17, wherein additively manufacturing the gauge comprisesprinting with a carbon loaded or silver loaded ink.